1. Field of the Invention
The present invention relates to an image forming apparatus, and in particular relates to an image forming apparatus including a recording head or print head to discharge ink droplets and a head tank to supply liquid ink to the print head.
2. Description of the Related Art
As an image forming apparatus such as a printer, a facsimile machine, a copier, a plotter, and a multifunction apparatus combining several of the capabilities of the above devices, an inkjet recording apparatus of a liquid discharging recording method employing a recording head formed of ink droplet discharging head is known.
Such an image forming apparatus has a print head that includes a head tank (which is referred to herein as a sub tank or a buffer tank) for supplying ink to the print head and a negative pressure generating mechanism that generates negative pressure in order to prevent exudation or leaking of the ink from the print head nozzles. The head tank of this type includes an ink container in which the ink is contained; a flexible member or film member to form part of the ink container; the negative pressure generating mechanism including an elastic member to press the flexible member outward; and an openably closable air release mechanism that exposes the interior of the ink container to the air. The ink is supplied from the ink container to the print head.
The head tank includes a displacing member (also known as a detector or a feeler) movable in response to the displacement of the flexible member. When the ink is supplied through air release filling from the main tank to the head tank in which air in the head tank is released, a carriage is moved to a predetermined detecting position (or a fill-up position) and a driver of the air release unit disposed on the apparatus body is operated so that the head tank is released to air, ink filling is performed from a state in which the carriage is moved to a predetermined position, and a position when the detector disposed at the apparatus body detects the displacing member is defined as a fill-up position.
As described above, when the displacing member that displaces in accordance with the amount of ink remaining is disposed at the head tank, and the ink is supplied from the main tank to the head tank, the carriage needs to be moved to a predetermined fill-up position. Supplying ink during printing when the amount of ink remaining inside the head tank becomes short necessitates interruption of the printing operation, which reduces the printing speed.
In such a case, amount of ink consumed of the head tank is calculated by counting the number of discharged ink droplets and the ink supply can be performed from the main tank to the head tank by an amount corresponding to the consumed amount. However, because the fill-up position is not correctly detected, an excess negative pressure due to the supply shortage or an excessively low negative pressure due to the excessive ink supply may be generated. Accordingly, the carriage needs to be positioned at the fill-up position regularly, the air release ink-filling needs to be performed, and the printing operation should be interrupted, thereby reducing the printing speed.
Provision of a detector of the amount of ink remaining or a driver for the air release unit at the side of the carriage and necessary members to control the ink supply to the head tank to be mounted to the carriage increase the weight and size of the carriage, thereby making the entire apparatus larger.